This invention relates to a method of obtaining needle-like single-crystals of polyoxymethylene directly from a monomer, trioxane, in a polymerization system and to the needle-like single-crystals of polyoxymethylene manufactured thereby.
The fact that trioxane polymerizes and gives high molecular-weight polyoxymethylene by cationic catalysis or radioactive irradiation is quite well known and various techniques for carrying out the polymerization process have been patented. In these patented techniques attention has been paid mostly to the productivity or the efficiency of the reaction and to the basic nature of the resultant polymer (e.g., to its average molecular weight). The structure of the polymer as polymerized has not been the main point of interest, because polymers are generally fabricated into the required shapes (fibre, film etc.) after they are produced.
It should be emphasized that the present invention is not aimed at improving conventional techniques but at growing, directly from the monomer, needle-like crystals of polyoxymethylene, which are readily useful in their own shape with no further processing. To this end, it is critically important to select and control conditions so that the chemical reactions (i.e., the initiation and the propagation of polymerization) should take place in exact harmony with the physical phenomena (i.e., the nucleation and the growth of crystals).
Below are mentioned a number of patented methods, more or less relevant to the present application. The novelty of the present invention over each patented method is clear from the accompanying comments.
U.S. Pat. No. 2,989,510 describes a method of polymerizing trioxane in non-polar solvents, e.g., cyclohexane, with boron trifluoride dibutyl-ether as catalyst. The method uses the monomer at fairly high concentrations (35% or above) and, after initiating the reaction by adding the catalyst, the polymerization is conducted at a temperature below or very close to the precipitation temperature of the monomer. The system is said to be anhydrous and the role of water, which is of vital importance in the present invention, is not mentioned. The method itself is very similar to that reported by Baccaredda et al (J. Polymer Sci. 44, 266 (1963)) and has been pointed out to be virtually the same as a catalytic solid-state polymerization (Iguchi et al, Brit. Polymer J. 3, 177 (1971)). The product is considered to bear a fibrous morphology similar to that obtained by gamma-ray induced solid-state polymerization (see below). A similar method is also described in U.S. Pat. No. 3,156,671 for the copolymerization of trioxane with a small amount of ethylene oxide.
U.S. Pat. No. 3,156,671 points out the effect of water acting in conjunction with boron trifluoride, and discloses a process for the polymerization of trioxane in solution. Cyclohexane is used as solvent, boron trifluoride is used as catalyst and the polymerization temperature is not permitted to fall below 60.degree. C. so that the reactants remain in liquid phase. At first glance, this appears similar to the method of the present invention. However, the patented method uses very high monomer concentration (trioxane/cyclohexane weight ratio, as large as 1.5 to 1), which is obviously too concentrated for the purpose of the present invention. Furthermore, the polymerization is a co-polymerization with ethylene oxide and the incorporation of such foreign units along the molecular chain, even if the amount is small, is hazardous for the formation of neat (defectless) single-crystals such as obtained by the present invention.
The so-called solid-state polymerization induced by radioactive irradiation is another popular means used to convert trioxane to polyoxymethylene (U.S. Pat. Nos. 3,242,063 and 3,438,883). A typical procedure comprises preparing of monomer crystals, introducing of randomly distributed reactive cites by gamma-ray irradiation, and causing the polymerization to proceed in the interior of the monomer crystals. This method gives rise to fibrous products consisting of numerous bundle-like crystallites (less than several tens of Angstroms in diameter), the major portion of which align along one particular crystallographic axis of the monomer and the minor portion of which align along another. The product may be a form of slender fibre but is classified as a multi-crystal which contains a considerable fraction of unreacted monomer. Removal of the monomer gives porous structures.
One object of the present invention is to provide a method for obtaining needle-like single-crystals of polyoxymethylene directly from trioxane.
Another object of the present invention is to provide needle-like single-crystals of polyoxymethylene which are hexagonal in cross-section and are constituted of molecular chains extended and aligned in the direction of the length with full three dimensional regularity.